Let N ⊂ R r be a lattice, and let deg : N → C be a piecewiselinear function that is linear on the cones of a complete rational polyhedral fan. Under certain conditions on deg, the data (N, deg) determines a function f : H → C that is a holomorphic modular form of weight r for the congruence subgroup Γ 1 (l). Moreover, by considering all possible pairs (N, deg), we obtain a natural subring T (l) of modular forms with respect to Γ 1 (l). We construct an explicit set of generators for T (l), and show that T (l) is stable under the action of the Hecke operators. Finally, we relate T (l) to the Hirzebruch elliptic genera that are modular with respect to Γ 1 (l).